1. Field of the Invention
The present invention relates to an organic-inorganic hybrid material used as an optically-anisotropic material, to a proton-conductive material utilized in energy devices and electrochemical sensors, and to a fuel cell.
2. Description of the Background
Recently, a direct methanol fuel cell (DMFC) has been proposed, in which methanol is used in place of hydrogen for fuel. This is expected to give high-capacity batteries for mobile devices that are substitutable for lithium secondary batteries, and is now much studied in the art.
The important functions of the electrolytic membrane (proton-conductive membrane) for solid polymer fuel cells are to physically insulate the fuel (e.g., hydrogen, aqueous methanol solution) fed to the anode, catalyst electrode from the oxidizing gas (e.g., oxygen) fed to the cathode, to electrically insulate the anode from the cathode, and to transmit the proton having been formed on the anode to the cathode. To fulfill these functions, the electrolytic membrane must have some mechanical strength and high proton conductivity.
In the electrolytic membrane for solid polymer fuel cells, generally used is a sulfonic acid group-having perfluorocarbon polymer such as typically Nafion®. The electrolytic membrane of the type has good ionic conductivity and has relatively high mechanical strength, but has some problems to be solved such as those mentioned below. Concretely, in the electrolytic membrane, water and the sulfonic acid group contained in the membrane form cluster channels, and protons move in the cluster channels via water therein. Therefore, the ionic conductivity of the membrane significantly depends on the water content thereof that is associated with the humidity in the service environment in which the cells are driven. For poisoning reduction in the catalyst electrode with CO and for activation of the catalyst electrode therein, solid polymer fuel cells are preferably driven at a temperature falling within a range of from 100 to 150° C. However, within such a middle-temperature range, the water content of the electrolytic membrane in the cells lowers with the reduction in the ionic conductivity thereof, and the expected cell characteristics could not be obtained. In addition, the softening point of the electrolytic membrane is around 120° C. and when the cells are driven at a temperature around it, then the mechanical strength of the electrolytic membrane is unsatisfactory.
On the other hand, when the electrolytic membrane of the type is used in DMFC, then it leads to the following phenomenon. Naturally, the membrane readily absorbs water and its barrier ability against the fuel methanol is not good. Therefore, methanol having been fed to the anode penetrates through the electrolytic membrane to reach the cathode. Owing to it, the cell output power lowers, and this causes a methanol-crossover phenomenon. For practical use of DMFC, this is one important problem to be solved.
Given that situation, there is a growing tendency for the development of other proton-conductive membranes substitutable for Nafion®, and some hopeful electrolytic materials have been proposed. For inorganic proton-conductive material, for example, known is proton-conductive glass. This is obtained through polymerization of tetraalkoxysilane in the presence of acid in a sol-gel process, and it is known that its humidity dependency is low in a high-temperature range. However, it is not flexible and is extremely brittle, and large-area membranes are difficult to produce from it. Therefore, the material is unsuitable for electrolytes for fuel cells.
For easy film formation based on the good characteristics of in organic material, one proposal is a nano composite material hybridized with polymer material. For example, there is proposed is a method of forming a proton-conductive membrane by hybridizing a polymer compound having a sulfonic acid group in the side branches, a silicon oxide and a proton acid (for example, see JP-A 10-69817, pp. 4-7; JP-A11-203936, pp. 6-10; JP-A 2001-307752, pp. 6-7). Another proposal is an organic-inorganic nanohybrid proton-conductive material that is obtained through sol-gel reaction of a precursor, organic silicon compound in the presence of a proton acid (for example, see Japanese Patent 3,103,888, pp. 4-7).
These organic-inorganic composite and hybrid proton-conductive materials comprise an inorganic component and an organic component, in which the inorganic component comprises silicic acid and proton acid and serves as a proton-conductive site and the organic component serves to make the materials flexible. When the inorganic component is increased so as to increase the proton conductivity of the membranes formed of the material, then the mechanical strength of the membranes lowers. On the other hand, however, when the inorganic component is increased so as to increase the flexibility of the membranes, then the proton conductivity of the membranes lowers. Therefore, the materials that satisfy both of the two characteristics are difficult to obtain. Regarding the methanol perviousness of the materials, which is an important characteristic of the materials for use in DMFC, satisfactory description is not found in the related literature.
JP-A 2003-157863 and EP 1223632 A1 disclose a hydrolyzable silyl group-having carbon atom-containing compound of the following formula, which is for proton-conductive membranes.(R3)3-mXmSi—CH2CH2—(C6H4)n—CH2CH2—SiXm(R3)3-mwherein R3 represents a group selected from CH3, C2H5 and C6H5; X represents a group selected from Cl, OCH3, OC2H5 and OC6H5; m indicates a natural number of at most 3.
JP-A2002-42550 describes a method of forming an in-plane oriented silica (silicon dioxide, SO2) meso-structure through sol-gel reaction of a silica precursor such as tetraalkoxysilane or tetrachlorosilane in a surfactant-containing solution. According to this, however, the inorganic component (silica) aggregates in the structure formed, and flexible membranes are difficult to produce.
Liquid crystal polyorganosiloxanes containing laterally linked mesogenic units on the main chains are described in Liquid Crystals, 2002, 29, 9, 1247-1250.
The present invention is to solve the problems noted above. Specifically, the first object of the invention is to provide a novel organic-inorganic hybrid material; the second object is to obtain a highly heat-resistant, proton-conductive membrane favorable for fuel cells; and the third object is to provide a proton-conductive membrane resistant to high-concentration methanol and having low methanol perviousness favorably for DMFC, and to provide a fuel cell that comprises it.